The present invention relates in general to a digital indicator, and pertains, more particularly, to a digital readout indicator that utilizes strain gages for the measurement of surface deviation from a reference measurement.
The device of the present invention has extensive use in making surface deviation measurements such as in the making of machine parts to detect any out-of-roundness or out-of-flatness of the product. In the prior art such as in the Woodworth, et al U.S. Pat. No. 3,924,458, strain gages have been used in connection with the measurement of pressures. However, in accordance with the present invention a new use is found wherein strain gages may be employed in connection with the measurement of a surface deviation. Present instruments used for such surface deviation measurements include dial-type indicators which are costly, time consuming to operate, not totally accurate, and not easily readable. Other prior art patents known at this time include U.S. Pat. Nos. 3,784,912; 3,978,399; 3,509,460; 3,501,696; 3,667,041; 3,070,786; 3,576,128; 3,910,106; 4,082,999; 3,589,457; 3,617,878; 3,826,983; 3,652,299; and 4,082,998.
Accordingly, it is an object of the present invention to provide an improved indicator for registering measurements in surface deviation.
Another object of the present invention is to provide an indicator for such measurements wherein the measurements of deviation are displayed in a digital format.
A further object of the present invention is to provide a digital indicator for the measurement of surface deviation with an arrangement that is relatively simple in construction not requiring complex circuitry such as shown in, for example, U.S. Pat. No. 3,784,912.
Still another object of the present invention is to provide a digital indicator for the measurement of surface deviation employing a simple zeroing technique and preferably employing the combination of a momentary switch with a hold circuit.
A further object of the present invention is to provide a digital indicator for the surface deviation wherein the indicator can be operated quite readily by even an unskilled operator requiring as the method of operation, simply positioning the probe or plunger against the surface with a sufficient pressure and operating the momentary switch to zero the indicator with all subsequent measurements indicating deviations from this desired reference surface.
To accomplish the foregoing and other objects of this invention there is provided an indicator for digitally displaying a measurement in surface deviation from a reference measurement. The indicator generally comprises a cantilever means having contact means preferably in the form of a plunger or probe for contacting the surface to be measured. Means are provided for supporting the cantilever means so that the contact means thereof is positioned in contact with the surface with at least a minimum predetermined pressure against the surface that is being measured. Strain gage means are associated with the cantilever means preferably in the form of a pair of strain gages attached to opposing surfaces of the cantilever means. From the strain gage means there is provided a control signal preferably coupled from the common connection between the pair of strain gages. The indicator device also includes digital meter and display means preferably in the form of a digital volt meter having its output coupled to a liquid crystal display or similar display. The digital meter and display means has differential inputs. The indicator device also includes a momentarily operable hold means preferably including a momentary switch and holding capacitor, with one side of this means coupled to one of the differential inputs. An amplifier or the like preferably couples the strain gage signal to the other side of the hold means and also directly to the other one of the differential inputs.
In accordance with the method of use of the device of this invention the plunger or probe is adjustable in its position relative to the surface being measured and in operation of the device this plunger is positioned with at least a slight predetermined pressure on the surface so as to cause the cantilever to move thereby causing in turn an unbalance of the strain gages. In accordance with the invention, different embodiments may be provided employing either a single strain gage or two strain gages or possibly even more than two strain gages. Any unbalance of the strain gages is amplified preferably by an operational amplifier and this initial unbalance signal is coupled to the digital meter and display means at one input thereof. Upon operation of the momentarily operable hold means the differential inputs to the digital volt meter are alike thus zeroing the meter and displaying a zero on the display. Upon release of the hold means one of the inputs to the digital volt meter is essentially held constant at the initial setting while the other input tracks the signal from the strain gage means so as to register any subsequent deviations detected during a scanning of the part, the surface of which is being measured for surface deviations.